Thiocyanate (or SCN⊖, referred to herein as ‘SCN’) is a major constituent in waste water produced by many industrial processes. Some industrial waste water, such as those from the steel industry, petrochemical industry and water (effluent) produced through the gasification of coal, often contains a significant concentration of SCN. SCN is a common cyanide reaction product of industrial waste water (or effluent) where cyanide (CN⊖, referred to herein as ‘CN’) is used.
While SCN, at concentrations up to 0.1 mg/L, is often not regarded as being toxic, it can be converted to the highly toxic and volatile cyanogen chloride (CNCl) at high pH. Also, oxidation of SCN may release toxic hydrogen cyanide gas (HCN). At concentrations of 50 mg/L SCN has been reported to significantly interfere with the biochemical oxygen demand (BOD) of water systems having an adverse affect on aerobic aquatic organisms.
The thiocyanate anion is often produced naturally when industrial CN comes into contact with a natural source of sulfide. The main occurrence of such a process is where CN is used as an extractant in mining processes such as in the extraction of precious metals such as silver and gold from their respective ores.
As the toxicological and physiological importance of thiocyanate has become more and more evident and understood, there is an increasing interest in the removal of this anion from, in particular, industrial waste water. One of the known treatments involves the degradation of thiocyanate by microbiological oxidation (with use of, for example, Thiobacillus thioparus). However the degradation conditions are finely balanced and the treatment may be very expensive and complex. A further method of removing thiocyanate ions from waste water involves solvent extraction, which is based on the principle of extraction of thiocyanate ions into another immiscible phase (generally organic). Solvent extraction of thiocyanate ions with quaternary ammonium salts has been reported and employed on an industrial scale, however the environmental and safety-related drawbacks of this method are numerous. For example, the large quantities of flammable and toxic solvents required present a considerable chemical hazard. Also, the extractants themselves are often toxic, corrosive and cause long term adverse effects if released in the environment.
The present invention seeks to overcome at least some of the shortcomings of the known SCN extraction processes.